This invention relates to an angle sensor utilizing light.
According to a conventional method of detecting an rotation angle, a shaft of a potentiometer is rotated in accordance with a rotation angle of an object to be measured for detecting the rotating angle based on the variation of the resistance value of the potentiometer caused by the rotation. According to this method, however, due to the sliding friction of the potentiometer where the torque applied to the potentiometer is small, a large error occurs. For example, in a method of detecting the inclination angle by measuring the rotaton angle of a pendulum, for the purpose of decreasing the error, it is necessary to make sufficiently large the rotational moment of the pendulum so that it is difficult to construct such device to have a small size and to obtain a desired response characteristic. Another defect of the method of utilizing the potentiometer lies in a short life caused by friction.
To eliminate these defects it is necessary to measure the rotating angle of an object without touching the same so that methods of utilizing magnetic field or light have been proposed.
The apparatus utilizing magnetic field is constituted by a sensor which senses magnetism such as a magnetic resistance element or a Hall element, but where such apparatus is used in an enviromnment in which external magentic field exists the measured value is influenced by the external magentic field thereby causing error. For this reason, in a certain case it is necessary to provide a magentic shield.
On the other hand, where light is used, it is only necessary to shield light for obviating the influence caused by external light, and this measure is much feasible than the magnetic shield. For this reason, it is more advantageous to use light than magnetic field to avoide the effect cause by external disturbance.
FIG. 1 shows a basic construction of a prior art angle sensor utilizing light. As shown, a luminous diode 1 acting as a luminous element and a phototransistor 2 acting as a light receiving element are disposed coaxially with a suitable gap therebetween. The luminous diode 1 and the phototransistor 2 are disposed in a casing, not shown. This apparatus is constructed such that when a semicircular or sector shaped light shielding plate 4 supported by a shaft 3 is rotated its straight side 4A intersects the optical axis. With this construction, as the rotation angle of the shaft 3 varies, the light quantity reaching the phototransistor 2 varies so that a signal corresponding to the rotation angle can be produced as shown in FIG. 2. Generally, however, the light receiving surface of th phototransistor is small, with the result that as the rotation angle varies only a little, the state changes from a range A in which the incident light is perfectly intercepted to a range B in which the incident light is not intercepted as shown in FIG. 2, thus saturating the output. Accordingly, a linear output characteristic can be obtained only in a very narrow region C. Where a phototransistor having a large light receiving area is used, the linear region (C in FIG. 2) can be increased to a certain extent, but such phototransistor is a special one so that its cost of manufacturing is high and not economy. Since it is impossible to make infinitely large the light receiving area the maximum detection angle was only 5.degree.-6.degree.. The shape of the output characteristic curve shown in FIG. 2 is inherent to the principle of the angle sensor shown in FIG. 1 so that this shape can not be changed as desired. For this reason, it has long been desired to provide an angle sensor utilizing light capable of obviating the defects of the prior art.